/Engine/Source/Runtime/Core/Public/GenericPlatform/GenericPlatformMisc.h
Ensure that there enough space in the persistent download dir. Returns true if there is sufficient space. If there is insufficient space and bAttemptToUseUI is true, the function will return false and the user may be prompted to clear space
static bool CheckPersistentDownloadStorageSpaceAvailable ( uint64 BytesRequired, bool bAttemptToUseUI )